Derivative Images are deleted

I've got a problem with the Image or the Image Assist module. I don't know exactly....

I have one site example.com with several subdomains, for example one.example.com. When I edit a node an click on the "add-image" button (of the image assist module), I'm used to see all the uploaded images. But if I change the site to one.example.com, and click on "add-image", I get a message:

"[Image-name]'s has an unneeded thumbnail derivative images. The derivatives will be rebuilt to remove it.
[Image-name]'s has an unneeded preview derivative images. The derivatives will be rebuilt to remove it.
The derivative images for [Image-name] have been regenerated."

Well, the thumbnail was definitly NOT unneeded, it was used on the other page! And there is no thumbnail anymore, until I click once again on the add-image-button, where I can read:

"[Image-name]'s thumbnail derivative image was missing. The derivatives will be rebuilt to regenerate it.
[Image-name]'s preview derivative image was missing. The derivatives will be rebuilt to regenerate it.
The derivative images for [Image-name] have been regenerated."

This also happens the other way round, when I switch back to the other site.

Well, has anyone an idea, how I can fix this problem? I need Image-Assist on all the sites, but I cant use it with this settings....

Thanks a lot!
llkks